A non-intrusive optical technique was developed to provide time-resolved longitudinal and cross sectional images of the liquid
film in horizontal annular flow of air and water, revealing the interfacial wave behavior. Quantitative information on the liquid
film dynamics was extracted from the time-resolved images. The planar laser induced fluorescence technique (PLIF) was
utilized to allow for the optical separation of the light emitted by the film from that (more intense) scattered by the air-water
interface. The visualization test section was fabricated from a FEP (Fluorinated Ethylene Propylene) tube, which has nearly the
same refractive index as water, what allowed the visualization of the liquid film at regions very close to the pipe wall.
Longitudinal images of the liquid film were captured using a high speed digital video camera synchronized with a high
repetition rate Nd-YLF laser. Data sets were collected with sampling camera frequencies ranging from 250 to 3000 Hz. A
specially developed image processing algorithm was employed to automatically detect the position of the air-water interface in
each image frame. The thickness of the liquid film was measured at two axial stations in each processed image frame,
providing time history records of the film thickness at two different positions. Wave frequency information was obtained by
analyzing the time-dependent signals of film thickness for each of the two axial positions recorded. Wave velocities were
measured by cross-correlating the amplitude signals from the two axial positions. The results obtained allowed the verification
of the variation of the liquid film characteristics with global flow parameters, such as the liquid and gas flow rates. For the film
cross section observations, two high speed digital video cameras were used in a stereoscopic arrangement. The high repetition
rate laser had its laser sheet mounted so as to illuminate a pipe cross section. Images from the left and right cameras were
distorted by the use of a calibration target and an image correction algorithm. Distorted images from each camera were then
joined to yield the complete instantaneous cross section image of the liquid film. Comparisons with results from different
techniques available in literature indicate that the present technique presents equivalent accuracy in measuring the liquid film
properties. Time–resolved images of longitudinal and cross section views of the film are recorded, what constitute valuable
information provided by the technique implemented.

General Note:

The International Conference on Multiphase Flow (ICMF) first was held in Tsukuba, Japan in 1991 and the second ICMF took place in Kyoto, Japan in 1995. During this conference, it was decided to establish an International Governing Board which oversees the major aspects of the conference and makes decisions about future conference locations. Due to the great importance of the field, it was furthermore decided to hold the conference every three years successively in Asia including Australia, Europe including Africa, Russia and the Near East and America. Hence, ICMF 1998 was held in Lyon, France, ICMF 2001 in New Orleans, USA, ICMF 2004 in Yokohama, Japan, and ICMF 2007 in Leipzig, Germany. ICMF-2010 is devoted to all aspects of Multiphase Flow. Researchers from all over the world gathered in order to introduce their recent advances in the field and thereby promote the exchange of new ideas, results and techniques. The conference is a key event in Multiphase Flow and supports the advancement of science in this very important field. The major research topics relevant for the conference are as follows: Bio-Fluid Dynamics; Boiling; Bubbly Flows; Cavitation; Colloidal and Suspension Dynamics; Collision, Agglomeration and Breakup; Computational Techniques for Multiphase Flows; Droplet Flows; Environmental and Geophysical Flows; Experimental Methods for Multiphase Flows; Fluidized and Circulating Fluidized Beds; Fluid Structure Interactions; Granular Media; Industrial Applications; Instabilities; Interfacial Flows; Micro and Nano-Scale Multiphase Flows; Microgravity in Two-Phase Flow; Multiphase Flows with Heat and Mass Transfer; Non-Newtonian Multiphase Flows; Particle-Laden Flows; Particle, Bubble and Drop Dynamics; Reactive Multiphase Flows

A non-intrusive optical technique was developed to provide time-resolved longitudinal and cross sectional images of the liquid
film in horizontal annular flow of air and water, revealing the interfacial wave behavior. Quantitative information on the liquid
film dynamics was extracted from the time-resolved images. The planar laser induced fluorescence technique (PLIF) was
utilized to allow for the optical separation of the light emitted by the film from that (more intense) scattered by the air-water
interface. The visualization test section was fabricated from a FEP (Fluorinated Ethylene Propylene) tube, which has nearly the
same refractive index as water, what allowed the visualization of the liquid film at regions very close to the pipe wall.
Longitudinal images of the liquid film were captured using a high speed digital video camera synchronized with a high
repetition rate Nd-YLF laser. Data sets were collected with sampling camera frequencies ranging from 250 to 3000 Hz. A
specially developed image processing algorithm was employed to automatically detect the position of the air-water interface in
each image frame. The thickness of the liquid film was measured at two axial stations in each processed image frame,
providing time history records of the film thickness at two different positions. Wave frequency information was obtained by
analyzing the time-dependent signals of film thickness for each of the two axial positions recorded. Wave velocities were
measured by cross-correlating the amplitude signals from the two axial positions. The results obtained allowed the verification
of the variation of the liquid film characteristics with global flow parameters, such as the liquid and gas flow rates. For the film
cross section observations, two high speed digital video cameras were used in a stereoscopic arrangement. The high repetition
rate laser had its laser sheet mounted so as to illuminate a pipe cross section. Images from the left and right cameras were
distorted by the use of a calibration target and an image correction algorithm. Distorted images from each camera were then
joined to yield the complete instantaneous cross section image of the liquid film. Comparisons with results from different
techniques available in literature indicate that the present technique presents equivalent accuracy in measuring the liquid film
properties. Time-resolved images of longitudinal and cross section views of the film are recorded, what constitute valuable
information provided by the technique implemented.

non-uniform liquid film distribution around the pipe
perimeter that characterizes this class of flow has been one
of the main focuses of research in the literature, together
with the prediction of pressure losses along the pipe.
One of the key fundamental questions related to the
liquid film behaviour is the mechanisms that maintain the
film at the upper pipe wall, compensating drainage induced
by gravity. Several mechanisms have been proposed as
indicated, for example, in the works of Butterworth &
Pulling (1972) and Jayanti et al. (1990). These mechanisms
are (i) secondary gas motion induced by the
circumferentially varying film thickness, (ii) liquid
entrainment and re-deposition, (iii) wave spreading due to
the distortion of liquid film waves at the bottom of the tube
and (iv) pumping action associated with the flow of gas
over the disturbance waves,
Measurements of the wave structure of the liquid film
have been conducted for both, vertical and horizontal

Introduction

The literature on two-phase annular flow through
horizontal pipes displays a large body of work spanning
decades of research. This continued interest is probably a
sign of the relevance and complexity of this flow
configuration.
In horizontal annular flow the liquid flows as a thin,
non-uniform film around the tube walls, while gas flows in
the pipe core. The liquid film in this flow regime presents a
wavy structure formed by ripples on the base film moving at
low velocities and larger and faster disturbance waves.
Droplets entrained in the gas flow are also part of liquid
transport. In process and transport pipes, as well as in heat
exchangers, the loss of liquid film coverage on the pipe wall
may result in accelerated corrosion rates or poor heat
transfer rates, increasing the probability of pipe wall failure.
The measurement and prediction of the time-varying,

developed is presented, together with results of experiments
conducted in a test section specially constructed for this
purpose. Air and water flow rates were varied in a range
limited by the test facility, but sufficient to reveal that the
technique produces results of comparable accuracy with
more established techniques. Contrary to other techniques,
such as electric-based probes, in the present approach every
image frame used to extract quantitative information from
the liquid film is also available for visualization, what
contributes to a better insight into the flow dynamics.

Experimental Facility

The present study employed non-intrusive optical
techniques to provide time-resolved images of the liquid
film in horizontal annular air-water flows. Two different
setups were utilized, one for capturing side views of the
lower portion of the liquid film and a second setup for
obtaining instantaneous images of the complete cross
section at a particular axial position of the liquid film. In
both cases, the planar laser induced fluorescence technique
- PLIF was employed to allow for the optical separation
of the light emitted by the film from that (more intense)
scattered by the air-water interface (Rodriguez & Shedd,
2 4,.~~ Rhodamine B at a concentration of 500 pug per liter
of water was employed as the fluorescent material. The
fluorescent material was excited by a sheet of green light
(527-nm wavelength) emitted by a double cavity,
high-repetition rate, Nd-YLF laser.
The liquid film in annular horizontal flows is expected
to display small thicknesses, ranging from a few
micrometers to thousands micrometers. Optical distortions
due to the mismatch between the indexes of refraction of the
tube wall material and the liquid, preclude the correct
visualization of the film thickness. For this reason, the test
section designed for the experiments employed pipes
fabricated from FEP (Fluorinated Ethylene Propylene),
which has nearly the same refractive index as water, what
allowed for the visualization of the liquid film at regions
close to the pipe wall (Hewitt et al., 1990).
Figure 1 presents a schematic view of the test section
utilized in the experiments for measuring the liquid film at
the lower part of the tube. Water from a progressive cavity
pump was fed to a 15-mm-diameter and 255-diameter-long
FEP tube. Air was supplied to the test section by a
centrifugal compressor. Air and water were mixed at a tee
connection located at the inlet section of the tube. Calibrated
rotameters were used to measure the air and water flow rates.
The air-water flow exiting the tube was directed to a
centrifugal separator from where the water was returned to
the pump inlet, while the air was vented out of the
laboratory space. A rectangular visualization box was
installed at a distance of 190 diameters from the inlet. The
box was filled with water in order to minimize optical
distortions due to the pipe wall curvature.
A Pegasus dual-cavity, Nd-YLF, high-repetition laser
provided illumination of a longitudinal section of the tube. A
pair of cylindrical and spherical lenses was used to
transform the circular beam into a planar light sheet with
dimensions of 20 mm wide by 0.5-mm thick. The horizontal
light sheet coming from the laser was deviated by a 450
mirror so that the light entered vertically through the bottom
wall of the visualization box and passed through the FEP

annular flows for many years. These measurements include
local time variation of film thickness, wave velocity and
frequency, as well as spectral properties of film thickness
time records. In these studies, resistance probes (Jayanti et
al., 1990 and Paras & Karabelas, 1991) and, more recently,
optical methods were employed (Shedd & Neweel, 1998).
Flow visualization has also been used as a tool to aid in
characterizing the film wave behaviour. Its different
implementations have followed the development of image
technology along the years. Cine movie with steady external
illumination was used in conjunction with dye injection by
Taylor & Nedderman (1968) and by Butterworth & Pulling
(1972). Later, high speed video systems were introduced
and replaced the time-consuming movie processing (Hewitt
et al., 1990). Sutharshan et al. (1995) employed the
photochromic dye activation technique to generate fluid
tracers within the liquid film that were followed by
high-speed digital imaging equipment with external back
lighting. The analysis of the digital images provided
qualitative information on the effects of wave passage on
the liquid axial and circumferential velocity in the film.
Typical liquid films in horizontal annular flow range
from a few micrometers to a few millimetres in thickness.
Visualization of such small dimensions at the
neighbourhood of a solid wall is a challenge for optical
techniques. Hewitt et al. (1990) used a tube material that
presented nearly the same index of refraction as water, what
allowed for the clear visualization of the film structure close
to the wall. Rodriguez & Shedd 1, 2 41) employed the same
tube material in a visualization setup based on the planar
laser induced technique PLIF. In this technique a
fluorescent dye dissolved in water and excited by a pulsed
planar sheet of laser light is used to produce clear images of
the air-water interface. The reflections from the interface are
blocked by an optical filter placed in front of the digital
camera, producing clear instantaneous images of
longitudinal sections of the film. Attempts to measure liquid
velocity within the film were made by Vassalo (1999) using
hot film probes in vertical annular flow. More recently,
Koplin (2 4II) obtained partial success with particle image
velocimetry (PIV) and particle tracking techniques to
estimate velocity field in the film.
The present work is part of an ongoing research project
aimed at providing simultaneous, time-resolved qualitative
and quantitative information on the liquid film structure in
horizontal annular flows. The experimental technique
implemented builds on the previous works of Hewitt et al.
(1990) and Rodriguez & Shedd 1, 2 41) in the sense that
employs index of refraction matching techniques associated
with PLIF. High frame rate cameras synchronized with
high-repetition rate lasers were used to provide
time-resolved, quality images of longitudinal and cross
section views of the liquid film around the pipe perimeter,
In the case of cross section views, two cameras were used in
a stereoscopic arrangement. Calibration targets were used to
distort the images obtained from the side viewing of the
pipe cross section. Instantaneous images of the film were
processed to enhance contrast and to extract the time
dependent properties of the liquid film such as, film
thickness, time-averaged and RMS values of film thickness-
frequency power spectra, wave velocity, and histograms of
film thickness distributions.
In the next sections a description of the technique

Cross sectional views of the liquid film were obtained
by an optical setup employing two high frame rate cameras
mounted at an angle, as indicated in Figure 3(a). In this case,
the light sheet was rotated by 900 so as to illuminate a cross
section of the pipe. Two IDT Motion Pro X3 cameras were
mounted at an angle of 450, imaging the pipe cross section
through the two 45-degree-inclined windows provided at the
visualization box that surrounded the test tube. Each camera
was mounted on a support that permitted that the camera
body were rotated in relation to the lens axis. This setup
allowed the attainment of the Scheimpflug condition. When
this condition is attained, the whole image is focused, even
though the camera is viewing the pipe cross section at an
angle (Raffel et al., 2007)

pipe, illuminating a longitudinal section of the air-water
flow inside the pipe. As will be commented in the results
section of the present paper, this optical setup produced a
non uniform illumination, with the liquid film in the lower
part of the pipe receiving more intense illumination than the
film in the upper portion of the pipe.

Figure 1: Schematic view of the test section.

f

Images of the lower portion of the liquid film were
captured using an IDT Motion Pro X3 camera operating
from 250 to 3000 frames per second at a spatial resolution
of 512 x 512 pixels. The camera was mounted orthogonally
to the light sheet plane. Nikkor lenses with focal distances
of 60 and 105 mm equipped with spacer rings were
employed, respectively, for the film thickness and wave
speed measurements. A TSI 610035 synchronizer was
employed to synchronize laser firing and image capture. At
the spatial resolution employed, the camera memory
allowed 52 seconds of image capture at 250 frames per
second and for 4 seconds at 3000 frames per second. A high
pass optical filter with a cutoff wave length of 560 nm was
installed in front of the camera lens to block the 527-nm
green laser light scattered by the air-water interfaces. With
the filter installed, the camera only registered the 610-nm
fluorescence light emitted by the Rhodamine dissolved in
the water what allowed for a clear visualization of the liquid
film. Figure 2 presents a detailed view of the optical setup.
Pixel calibration of the images was obtained by using a
target inserted into the test pipe through its exit section, after
the removal of the return pipe connection. The target was
machined from a 1-m-long brass cylinder. A length of fifty
millimeters at the extremity of the cylinder that penetrated
the pipe had half of its diameter removed by a machining
operation, so as to form a plane section passing through the
cylinder diameter. On this plane section, a grid of regularly
spaced vertical and horizontal lines was inscribed forming
the calibration target. After the target plane was aligned with
the vertical laser light sheet, the test pipe was filled with the
same water and Rhodamine solution used in the tests. An
image was then captured by the camera and the pixel
calibration calculated by measurements made with the
image acquisition software and the knowledge of the actual
grid spacing. It should be mentioned that measurements
made in the image at regions close to the lower pipe wall, at
the pipe center line and close to the upper pipe wall, all
presented the same pixel calibration value, indicating that no
appreciable optical distortions were present,

I
camera

Figure 2: Optical setup for longitudinal film visualization.

Figure 3: (a) Top view of optical setup for cross section
liquid film visualization. (b) Calibration target, as imaged
by left and right cameras and after application of distortion
procedure and joining operation.

Image distortion due to the side camera viewing was
corrected by a specially written program that used images of
a cylindrical target captured by the left and right cameras.
The target, shown in Figure 3(b), was machined from 1-m
long cylindrical brass piece, and was introduced in the test
tube through its exit section, in the same fashion as
described before for the side view experiments. A grid of
regularly-spaced dots was machined on the target face
which was black anodized. The dots were afterwards
painted white to enhance contrast. As part of the calibration
procedure, the target was inserted into the FEP test tube and
had its face aligned with the laser light sheet plane. The test
tube was filled with the Rhodamine-water solution and one
image of the target was captured with each camera and input
to the distortion routine developed. The routine provided a
distortion calibration polynomial for each camera that was
later applied to each flow image captured. The pixel
calibration value was also provided by the calibration
procedure. Figure 3(b) shows the target images as captured
by the left and right cameras and after the application of the
distortion procedure and joining operation that will be
described in the next section.

It is relevant to mention that the calibration conditions
described using a pipe completely filled with the
water-Rhodamine solution are distinct from those at flow
conditions when a liquid film is flowing at the wall and
there is an air core carrying liquid droplets. Due to this
difference in optical paths, the image obtained from one
camera can not be used to image the liquid film on the
opposite pipe wall. As will be commented shortly, the right
part of the liquid film was captured by the right camera,
while the left part of the film was captured by the left
camera. With this separation it is guaranteed that the
calibration polynomial is always applied within a liquid
layer with no air layers in the optical path.

Image Processing

The image processing procedures employed in
extracting the film thickness versus time information from
the set of captured images will now be outlined.

Longitudinal images of lower liquid film. After a sequence
of images of the lower liquid film for a particular
combination of water and air flow rates was captured and
stored, the first step in the image processing procedure was
to specify the location and width of the two probes in the
image where the thickness of the liquid film would be
measured. Figure 4 was prepared to aid in the definition of
the measuring probes in each image frame.

Figure 4: Schematic view of the probes employed for
measuring the time-varying liquid film thickness.

The figure is a pictorial view of a captured image frame
where the lower and upper tube walls can be seen. The grid
in the figure background corresponds to the camera pixels,
while the curve represents an idealized instantaneous liquid
film geometry captured by the camera. The two vertical
strips marked in the figure are the selected probe regions
where the film thicknesses are to be measured. The probes
are spaced by a distance ds with widths given by 1,; and Is2.
In the figure, the thicknesses measured in probes 1 and 2,
corresponding to the time of capture t, are h;(t) and h2(t).
The distance between the probes influenced the
uncertainty on the wave speed measurements, while the
probe width determined the level of spatial averaging to be
applied to the thickness data. In order to keep the
experimental uncertainty levels within acceptable limits, the
experiments for measuring film thickness and wave speed
were conducted separately using different pixel resolutions
and acquisition frequencies. For the film thickness
measurement experiments the optical system used a
105-mm lens, which produced a pixel resolution of

20pum/pixel. The probe width employed was equal to 50
pixels, which is equivalent to 1 mm in the flow domain. For
the wave speed measurements, a 60-mm lens was employed
producing a resolution of 55 pum/pixel. The probe spacing
selected was equal to 150 pixels, which is equivalent to
8.25mm in the flow domain. In these experiments the probe
widths were equal to 37 pixels or 2 mm. It should be
mentioned that successful results were also obtained with
much smaller probe widths, but are not reported here.
Prior to measuring the film thickness data, the images
were processed with the objective of enhancing contrast. It
was verified that the original liquid film images captured
presented narrow grey level ranges that did not span the full
dynamic range offered by the camera. Besides,
imperfections in the tube wall material and spatial variations
on the light sheet intensity produced variations in the grey
level of the film images. These image characteristics were
corrected by a histogram equalization procedure based on a
sigmoid function that transforms the grey levels above and
below a pre-determined value to, respectively, the maximum
and minimum pixel grey values available in the camera
sensor. The resulting image grey level histogram displays
higher number of pixels concentrated on the extreme high
and low values, with fewer number of pixels displaying
intermediate values. This type of image histogram facilitates
the determination of a threshold for the binarization
operation that follows. A distinct feature of the histogram
equalization procedure employed in the present study was
its application to each individual image column, rather than
to the whole image.

Figure 5(a) presents a sample of a typical original
instantaneous liquid film image captured by the camera.
Below the image is the corresponding histogram. As can be
verified, the number of pixels with values above 0.7 is
negligible, with the majority of pixels concentrated in the
zero-to-0.6 range. Figure 5(b) shows the effect of the
column-based histogram equalization procedure applied
over the original image. The image contrast has been
significantly enhanced and a binarization operation can be

and the film amplitude measurement was performed with
the same procedure used for the longitudinal measurement
already described. The amplitude measurements in the cross
stream images were always made at the Oo position (bottom
part of the film). Film amplitudes at other circumferential
positions were obtained by rotating the cross section image
to the Oo position by applying a rotation transformation to
the image.

easily implemented,
Figure 6(a) presents the image resulting from the
binarization operation using an appropriate threshold value.
The liquid film thickness at a determined axial position in
the binary image can be easily determined by counting the
number of white pixels until the first black pixel is found
(interface), once the position of the lower wall is input to the
program. In Figure 6(b), as a verification procedure, a red
line corresponding to the measured film thicknesses is
overlaid on the original image (Figure 5(a)). The agreement
obtained is considered excellent. The white patches over the
liquid film, but not connected to it, are images of fluid out
of the illumination plane, and we were not computed in the
film thickness measurement.
It should be mentioned that the image processing
procedures just described were not applied on the entire
images, as suggested by the examples shown in Figures 5
and 6. Rather, in order to save processing time, the image
processing procedures were applied only on the regions
defined as probes 1 and 2.

Ih~

Figure 6: (a) Binary image. (b)
amplitude ovelaid on original image.

Measured liquid film

Cross section images of liquid film. The processing
procedure applied to the liquid film images captured by the
right and left cameras was initiated by applying a histogram
equalization procedure to the image pair. This was necessary
since the two images presented different grey level
distributions due to differences in illumination. Here,
contrary to what was previously described for the
longitudinal film images, a global histogram procedure was
applied to the images. Following, the distortion calibration
polynomials that were previously determined with the aid of
the calibration target were applied to each image. The
resulting distorted left and right images can be seen in
Figure 7(b). Figure 7(a) presents the original left and right
images, before distortion.
Next, the two images were joined to form a complete
instantaneous image of the film cross section. In order to
assure a perfect matching of the left and right images, the
position of the center of the calibration target as viewed by
each camera was recorded during the calibration operation.
These positions were used as a guide to match the images.
Figure 7(c) presents the result of the image joining operation.
In this figure, a circular black mask was applied to the
exterior of the pipe to trim ghost images resulting from film
images out of the laser sheet and viewed by the cameras
through the transparent tubes walls. This was merely a
cosmetic operation with no implication on the film
amplitude measurements. Figure 7(d) presents the joined
images with the overlaid mask. The images were binarized

Figure 7: (a) Liquid film images as captured by the left and
right cameras. (b) Distorted left and right images. (c) Joined
images. (d) Joined images with overlaid mask.

Wave Characteristic Measurements

Quantitative information on the liquid film wave
behaviour was extracted from the thickness versus time data
measured from the longitudinal and cross section time
resolved images of the liquid film.
As already mentioned, liquid film thickness versus time
and wave velocity data were obtained from separate
experiments employing different pixel calibration values
(different optical magnification) and different acquisition
frequencies. For the amplitude versus time data, an
acquisition frequency of 250 Hz was employed. The record
was limited to 52 s by the camera memory. Time-averaged
liquid film thickness was obtained from the amplitude data
by averaging the amplitude data over the record length. This
value includes the contributions of large amplitude waves,
as opposed to the data reported by Schubring & Shedd
(I ne I ',i that only considers the base film thickness variation.
Root-mean-square (RMS) values of the film thickness data
were also calculated to aid in the flow characterization.
Power spectra densities (PSD) of the film thickness

time records were calculated employing 256 Hamming
windows to filter the results (Harris, 1978) that would,
otherwise, be too noisy due to the camera memory
limitation.
The film thickness data obtained by the longitudinal
camera viewing, gives a 512 x 512 pixel resolution was
sufficient to yield acceptable accuracy in the thickness
measurements, since only the region ranging from the
bottom wall to the pipe centreline was imaged by the
camera. For this pixel resolution the camera memory
allowed the already mentioned record length of 13100
images, which at 250 Hz translated to 52 s of recording time.
In the case of the stereoscopic cross section data however,
each camera images half of the tube cross section, which
requires the use of the 1024 x 1280 maximum pixel
resolution offered by the camera in order to guarantee an
acceptable accuracy in the measurements. With this pixel
resolution, the camera memory allows capturing 6550
images, which correspond to a maximum recording time of
26 s at 250 Hz acquisition frequency. Due to the shorter
record length, the PSD for the stereoscopic cross section
data were calculated employing 128 Hamming windows to
improve the data filtering.
Cross correlation function (CCF) of the time-dependent
liquid film thickness records measured at the locations of
probe 1 and 2 were calculated to estimate the wave velocity
(Bendat & Piersol, 1971). The wave velocity was obtained
by dividing the distance between probes 1 and 2, ds, by the
time corresponding to the cross-correlation peak.
Preliminary tests conducted indicated that the camera
acquisition frequency should be increased to 3000 Hz so
that the cross correlation results provided acceptable
accuracy for the wave velocity data.
The statistical calculations just described were
implemented using routines available in the Matlab
software.

Results and Discussion

An experimental program was conducted with the
objective of validating the optical technique developed. The
experiments covered the ranges of water and air flow rates
indicated in Table 1. These operational conditions were
chosen so as to allow comparison with results available in
the literature. According to Taitel & Dukler (1976) flow map,
these flow conditions are all within the annular flow regime
region.
All results to be presented were obtained with the
15-mm internal diameter pipe described in the experiments
section, operating at atmospheric pressure level.

Longitudinal and cross section visualizations. Since the
technique developed is based on the digital processing of
time resolved images of the liquid film, high quality
instantaneous images of longitudinal and cross section
views of the liquid film were registered and available for
analysis. An examination of these images provides valuable
visualizations of the dynamics of the films. Although in the
present paper only the probe regions defined in each one of
the images were analysed for quantitative data extraction,
samples of the complete longitudinal and cross stream
images are displayed in Figures 8 and 9.

Figure 8: Time-resolved longitudinal views of the liquid
film at the bottom of the tube.

In the case of the longitudinal images, Figure 8, the
sample presented is part of a set of 13100 images of the film
captured at the bottom of the tube. The images shown in the
figure were captured at 3000 frames per second and display
the passage of a liquid wave. Also seen in the figure are two
marks, one red one blue, at the air-water interface. These are
the film thicknesses measured at the two probe locations by
the image processing procedures. These marks were
overlaid on the original images to serve as a visual
verification of the accuracy of the image processing
procedures employed. It should be mentioned that the visual
observation of the level of agreement of the red and blue
marks with the all original images captured was part of the
experimental procedure. After the complete set of images
was recorded and processed, a movie with all the images

illumination due to the wider light sheet necessary to
illuminate the complete cross section of the pipe. It should
be mentioned that successful tests were conducted at
2000Hz. The image sequence selected presents the passage
of a large disturbance wave formed under the conditions of
superficial air and liquid velocities of, respectively, 20 m/s
and 0.140 m/s. A visual analysis of a slow motion
sequence of these images allows the observation of the
circumferential motion of the liquid film climbing on the
tube wall. As can be verified in the figure, the laser
incidence from below resulted in poor illumination of the
upper part of the tube, which made upper tube film
thickness measurements less accurate.
The optical setup is presently being modified to
produce uniform illumination that will permit measurements
at the upper tube region. For comparison purposes, Figure
9(b) presents longitudinal images of a disturbance wave
formed for the same conditions as those indicated for the
cross stream view. It should be mentioned that the images
presented in Figure 9(a) and (b) were different realizations
of the experiment at the same flow conditions and were not
captured simultaneously, as the side-by-side presentation
tends to suggest.

Time-resolved liquid film thickness. Figure 10 presents a
sample of time records of the film thickness at the bottom of
the pipe obtained by analysing the captured longitudinal
image sequences by employing the image processing
procedures developed. The results of Figure 10 correspond
to the film thickness measured at the location of probe 1
defined in Figure 4. Figure 10 presents the film thickness
for a constant liquid superficial velocity and for different
values of the air superficial velocity. The presence of
disturbance waves and ripples can be identified in the
records presented. Also, the results show the decreasing
trend of the film thickness and regularization of the large
waves as the superficial gas velocity increases, as pointed
out by several authors (Jayanti et al., 1990 and Paras &
Karabelas, 1991). The results found in the literature were
obtained employing electrical probes.

Time-averaged and RMS film thickness. Average film
thicknesses at the bottom of the pipe obtained from the
measured time records for all operating conditions
investigated are presented in Figure 11. The results
indicate that the average liquid film at the bottom of the
tube is a decreasing function of the superficial gas velocity,
depending weakly on the superficial liquid velocity for the
range of flow rates investigated. These trends agree with
previous results available in the literature (Jayanti et al.,
1990 and Paras & Karabelas, 1991)
RMS values of the film thickness at the bottom of the
tube can also be extracted from the time-resolved thickness
data obtained. In Figure 12, the ratio of the RMS thickness
data to the time-averaged thickneSS, h/h is plotted as a
function of the superficial gas velocity, for different values
of the superficial liquid velocity. This ratio is a measure of
the intensity of the film thickness fluctuation, and reaches
its peak of 67% for the conditions corresponding to the
lowest gas velocity (20 m/s) and intermediate liquid
superficial velocity (0.084 m/s). These results are in
agreement with the work of Paras & Karabelas (1991) that

captured containing the red and blue dots was observed,
Small deviations from the measured and original positions
of the interface could be easily identified by eye. A large
number of frames presenting deviations would be an
indication of bad image quality or of a bad choice of image
processing parameters. The number of small deviations
encountered in the experiments, like those shown in Figure
8, was negligible.

Figure 9: Time-resolved images of disturbance wave for
u =20 m/s and ugl=0. 140 m/s. (a) cross section views of the
li uid film at 250 frames per second. (b) longitudinal views
at the bottom of the tube at 250 frames per second.

Figure 9(a) presents a sample of cross section views of
the liquid film around the tube perimeter captured at 250
frames per second. Contrary to the longitudinal view just
presented, in this case the camera frame rate had to be
lowered to cope with the lower intensity of the laser

Figure 12: Ratio of the RMS liquid film thickness to the
time-averaged thickness.

Wave velocity results. Wave velocity measurements were
obtained by cross correlating the time-resolved thickness
data measured at the location of probes 1 and 2. A typical
cross correlation function is presented in Figure 13, where
the abscissa indicates the number of image frames. The
corresponding time lag can be obtained dividing the number
of frames by the camera acquisition frequency. In all tested
cases, a 3000 Hz acquisition frame was utilized. In the
OXample of Figure 13, the correlation peak is found at 4.7
frames, which corresponds to a time interval of 1.57 ms.
The wave speed is obtained by dividing the probe distance,
d,= 8.22 mm, by this time interval yielding a wave speed of
5.24 m/s.

100

0 95

S0 90

00 02 04 06 08 1
Time (s)

0 80

-20 -15 -10

0
Frames

10 15 20

Figure 10: Time records of liquid film thickness at the
bottom of the pipe obtained by analysis of longitudinal
images. Data for usz=0. 112 m/s and superficial gas velocities
equal to (a) usg=20 m/s (b) usg=28 m/s and (c) us,-34 m/s.

Figure 14 presents the wave velocity values obtained
for all the experiment conducted. In this figure, measured
wave velocities are plotted as a function of the superficial
gas and liquid velocities. Not only was the correct
dependence of the wave speed with liquid and gas
superficial velocities captured by the optical technique
developed, but also the numerical values obtained are in
gOod agreement with the results from Schubring & Shedd
(I III and Fukano & Ousaka (1989).

0700

E 06000

0400

O 300

Figure 11: Time-averaged liquid film thickness at the
bottom of the tube.

200 ,,,

Figure 14: Wave velocity measured at the bottom of the
tube.

Power spectra density (PSD) of thickness time records.
Figures 15 and 16 present PSD plots of the time-resolved
thickness data for the flow conditions indicated. These
PSD results were selected among all the flow conditions
tested as an example to convey the capability of the optical
technique implemented to extract spectral information
from the film thickness data.

Figure 15 was prepared to reveal the influence of liquid
superficial velocity on frequency distribution of the film
thickness time variation. The top and bottom figures
correspond to two different values of the superficial gas
velocity, namely 20 and 28 m/s. The peaks in the PSD
indicate the dominant frequency of the thickness time
signal and are seen to be a weak function of the liquid
superficial velocity for the lower gas flow condition
presented in Figure 15(a). Also, for this gas flow condition,

the strength of the peaks for each liquid superficial
velocity presents the same order of magnitude, indicating
that the energy level associated with the film waves are
approximately the same, for the flow conditions
represented in Figure 15(a). As the superficial gas velocity
is increased, Figure 15(b), the dominant frequencies are
seen to increase with the superficial liquid velocity. This
trend is contrary to that reported in the works of Paras &
Karabelas (1991) and Jayanti et al. (1990), but it is in
agreement with the results of Schubring & Shedd (pn ly~.
The vertical spreading of the PSD curves for the higher
superficial gas velocity indicates that film waves related
with the higher superficial liquid velocities are associated
with higher energy levels.

The presentation of Figure 16 conveys the influence of
the gas velocity on the PSD of the film thickness time data
for two values of the liquid superficial velocity, namely,
0.084 and 0.112 m/s. The influence of the gas superficial
velocity is significant for both liquid flow values. A clear
increase in the dominant frequency with the superficial gas
velocity is observed for both superficial liquid velocities, a
trend also reported in the literature (Jayanti et al., 1990 and
Paras & Karabelas, 1991)

Film thickness histogram. The measured time-varvmng liquid
film thickness data can be used to construct histograms
displaying the probability of occurrence of the different
peaks in film thicknesses. One such histogram is presented
in the traditional bar format in Figure 17 for the test case
characterized by u =20 m/s and usi=0. 112 m s. An analysis

in which side view images of the bottom part of the film are
captured and processed. In the second method, stereoscopic
images of the cross section of the liquid film are obtained
and processed to yield quantitative information on the film
statistical and spectral properties around the tube perimeter.
At the time of preparation of the present manuscript, the
issue with the uneven illumination on the tube cross section
mentioned previously had not yet been resolved, what
precluded the realization of measurements in the upper
portion of the tube cross section. Measurements could be
made, however, at the bottom part of the tube where laser
illumination was satisfactory. The results obtained are
compared with those obtained by the longitudinal technique
and already presented in this results section.
Figure 20 presents the PSD obtained by the
longitudinal and stereoscopic techniques for usg 20 m/s and
usi=0.140 m s. The agreement obtained is considered
excellent.

of the figure reveals that film thicknesses in the 0.5 to
1.5-mm range comprise the majority of the thickness
amplitudes present in the flow.

In order to allow for an assessment of the effects of the
gas and liquid flow rates on the film thickness distribution,
histograms for different flow operational conditions were
plotted in the same diagram. To avoid overcrowding the
figure, continuous lines were fit to the histogram bars, as
indicated in Figure 17. Figure 18 presents histograms of
film thicknesses for air superficial velocity equal to 20 m/s
and for different water superficial velocities. The figure
indicates that, for this value of the gas superficial velocity,
there is no significant effect of the liquid flow rate on the
film thickness distribution, with the majority of film
thicknesses found within the 0.5 to 1.5-mm range.

S1000 2000 3000 4000
h(mm)

use m/is,
-.- s'
0O 084
0O 112
0O 140

Figure 18: Liquid film thickness histogram for usg=20 m s.

The effect of the superficial gas velocity on the film
thickness distribution is clearly seen in Figure 19 where the
thicknesses histograms are plotted for liquid superficial
velocity equal to 0.112 m/s and for different values of the
gas superficial velocity. An inspection of Figure 19 reveals
that the thickness distribution changes significantly as the
gas velocity is increased from 20 to 34 m/s. Indeed, it can
be verified that the thickness distribution changes
markedly from 20 to 24 m/s, a gas superficial velocity
beyond which the majority of the thickness values are
tightly grouped around the 0.5-mm bin, which is an
indication of the regularization effect that the gas imposes
on the wave character of the liquid film.

.Stereo
* Longitudinal

srequency H1
Figure 20: Comparison of PSD of film thickess data at the
bottom of the tube obtained by the longitudinal and
stereoscopic optical techniques for usg 20 m/s and
usi=0.140 m s.

For the same flow conditions of Figure 20, the average
film thickness obtained by the longitudinal and stereoscopic
techniques were, respectively, 0.88 and 0.87 mm what
attests for the potential of the stereoscopic technique.

The present paper presented an optical technique
developed for measuring the statistical and spectral
properties of time-varying liquid film thicknesses in
horizontal, air-water annular flow. The technique proposed
builds on existing visualization techniques described in the
literature. It uses tube material with index of refraction that
nearly matches that of water in order to permit the
visualization of thin liquid films adjacent to the tube wall.
Laser induced fluorescence was employed to separate the
intense light reflected from the air-water interfaces and
allow the recording of the desired liquid film images.
Recording of the images was conducted with high frame
rate cameras what produced time-resolved data with good
spatial resolution. The quality of the film images obtained
allowed the visualization of the wave behaviour of the liquid
film.
Two versions of the optical technique were
implemented. In one technique, a longitudinal section of the
film defined by a pulsed laser light sheet is imaged with a
high frame rate camera synchronized with laser firing. The
second technique employed two high frame rate cameras in
a stereoscopic setup to yield an instantaneous view of the
complete cross section of the liquid film.
Specially developed image processing routines were
applied to improve image contrast, to calibrate the images,
and to correct for the distorted views associated with the
stereoscopic setup. Quantitative information on the
statistical properties of the liquid film was extracted from
the digital images for both, the longitudinal and stereoscopic
setups. The processing results included time-resolved film
thickness, time-averaged and RMS thickness values, wave
velocities and power spectra density of the thickness data.
The influence of superficial gas and liquid velocities on
these quantities was identified and compared with data
available in the literature obtained by different measuring
techniques. Good agreement was obtained with the data
from the literature, which served to validate the technique.
Histograms of the film thickness distribution were also
presented to complement the statistical characterization of
the liquid film time varying data.
Overall, the technique presented good results and can
potentially contribute to a better understanding of annular
liquid-gas two-phase flows.

Acknowledgements

The authors gratefully acknowledge the support
awarded to this research by Petrobras R&D Center. Our
gratitude is also extended to the Brazilian Research Council,
CNPq, for the scholarships and continued support to our
research activities. The authors also thank the enthusiastic
cooperation from undergraduate students Bruno Dreux and
Carlos Eduardo Correia. Special thanks go to laboratory
technicians Leonardo Pinhal and Juarez Felisberto.